1. Field of the Invention
The invention generally relates to output stages of electronic components. More particularly, the invention relates to output stages in which increased current sourcing capability is desired.
2. Related Art
FIG. 1 depicts a conventional output stage that is suitable for use as an output buffer in, for example, Class AB amplifiers. While possessing favorable power supply rejection ratio (PSRR) performance, rail-to-rail output swing, and current sinking capability, the FIG. 1 circuit has relatively weak current sourcing capability (IOUT is undesirably small).
Due to current mirroring phenomena in the FIG. 1 configuration, the maximum output source current IOUT is ultimately determined by how much gate-to-source voltage (VGS) can be applied to the MP2 sink. Assuming that all transistors in FIG. 1 are ideal and the same size, MN3 conducts IB when VIN is equal to VB, a bias voltage applied to MP1. When VIN swings an incremental voltage amount xcex94V below VB, current through MP2 increases accordingly. In the illustrated configuration, this MP2 current increment is mirrored to MP4.
Unfortunately, only part of the input change xcex94V in VIN is reflected as an increment in VGS for MP2. Because the gate-to-source voltage VGS for MN2 (connected above MP2) is fixed by the gate""s connection to the gate of MN1, part of the xcex94V is xe2x80x9cabsorbedxe2x80x9d by VGS of MN2 to account for the current increment. Thus, because the maximum output source current IOUT is ultimately determined by how much gate-to-source voltage (VGS) can be applied to the MP2 sink, IOUT does not adequately increase with xcex94V variations in VIN, resulting in undesirably small maximum output source current IOUT.
Accordingly, there is a need in the art to provide increased current sourcing capability of circuits such as that illustrated in FIG. 1, preferably with minimal additional circuitry and without compromising other operational features of the circuit.
Accordingly, there is provided an output stage for providing an increased current source output. The output stage has a first circuit element that carries a first current that varies in accordance with an input voltage and a feedback voltage that includes a first voltage across the first circuit element. The output stage also has a positive feedback arrangement that dynamically increases the feedback voltage in response to an increase in at least the first current, to supplement an increase in the first current beyond that caused by a change in the input voltage alone. The output stage also has a second circuit element that provides the increased current source output as a mirror of the first current.